1. Field of the Invention
The present invention relates generally to wireless communication systems and, more particularly, to the transmission of physical downlink control channels.
2. Description of the Art
A communication system includes a DownLink (DL) that conveys transmission signals from transmission points, such as Base Stations (BS or NodeBs) to User Equipments (UEs), and an UpLink (UL) that conveys transmission signals from UEs to reception points, such as NodeBs. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a cellular phone, a personal computer device, etc. A NodeB is generally a fixed station and may also be referred to as an access point or some other equivalent terminology.
DL signals consist of data signals conveying information content, control signals conveying DL Control Information (DCI), and Reference Signals (RS), which are also known as pilot signals. A NodeB transmits data information or DCI to UEs through respective Physical DL Shared CHannels (PDSCHs) or DL Control CHannels (CCHs). UL signals also consist of data signals, control signals and RS. A UE transmits data information or UL Control Information (UCI) to a NodeB through a respective Physical Uplink Shared CHannel (PUSCH) or a Physical Uplink Control CHannel (PUCCH). It is possible for a UE having transmission of data information to also convey control information through the PUSCH.
A PDSCH transmission to a UE or a PUSCH transmission from a UE may be in response to dynamic scheduling or to Semi-Persistent Scheduling (SPS). With dynamic scheduling, a NodeB conveys to a UE a DCI format through a respective Physical DL Control CHannel (PDCCH). With SPS, a PDSCH or a PUSCH transmission is configured to a UE by a NodeB through higher layer signaling, such as Radio Resource Control (RRC) signaling, in which case it occurs at predetermined time instances and with predetermined parameters as informed by the higher layer signaling. A SPS PDSCH or a PUSCH transmission can be activated or deactivated by a PDCCH.
A NodeB may also transmit multiple types of RS including a UE-Common RS (CRS), a Channel State Information RS (CSI-RS), and a DeModulation RS (DMRS). The CRS is transmitted over substantially the entire DL system BandWidth (BW) and can be used by all UEs to demodulate data or control signals or to perform measurements. To reduce the overhead associated with the CRS, a NodeB may transmit a CSI-RS with a smaller density in the time and/or frequency domain than the CRS for UEs to perform measurements and transmit a DMRS only in the BW of a respective PDSCH and a UE can use the DMRS to demodulate the information in the PDSCH.
FIG. 1 illustrates a conventional transmission structure for a DL Transmission Time Interval (TTI).
Referring to FIG. 1, a DL TTI consists of one subframe 110 which includes two slots 120 and a total of NsymbDL symbols for transmitting data information, DCI, or RS. The first MsymbDL subframe symbols are used to transmit PDCCHs and other control channels 130 (not shown) including a Physical Control Format Indicator CHannel (PCFICH) transmitted in the first subframe symbol and indicating the number MsymbDL and Physical Hybrid ARQ Indicator CHannels (PHICHs) informing UEs whether respective PUSCHs were correctly or incorrectly received. The remaining NsymbDL−MsymbDL subframe symbols are primarily used to transmit PDSCHs 140. The transmission BW consists of frequency resource units referred to as Resource Blocks (RBs). Each RB consists of NscRB sub-carriers, or Resource Elements (REs), and a UE is allocated MPDSCH RBs for a total of MscPDSCH=MPDSCH·NscRB REs for the PDSCH transmission BW. A time-frequency resource of one DL TTI and one RB is referred to as a Physical Resource Block (PRB). Some REs in some symbols contain CRS 150, CSI-RS or DMRS.
DCI can serve several purposes. A DCI format conveyed by a PDCCH may schedule a PDSCH or a PUSCH transmission. Another DCI format in a respective PDCCH may schedule a PDSCH providing System Information (SI) to UEs for network configuration parameters, or a response to a Random Access (RA) by UEs, or paging information, and so on. Another DCI format may provide to a group of UEs Transmission Power Control (TPC) commands for adjusting a respective transmission power for a PUSCH or a PUCCH.
A DCI format includes Cyclic Redundancy Check (CRC) bits in order for a UE to confirm a correct detection. The DCI format type is identified by a Radio Network Temporary Identifier (RNTI) that scrambles the CRC bits. For a DCI format scheduling a PDSCH or a PUSCH to a single UE, the RNTI is a Cell RNTI (C-RNTI). Also, a DCI format with a SPS-RNTI can activate or deactivate a SPS PDSCH or PUSCH transmission. A DCI format with a CRC scrambled by a C-RNTI or a SPS-RNTI will be referred to as providing UE-specific control information. For a DCI format scheduling a PDSCH conveying SI to a group of UEs, the RNTI is a SI-RNTI. For a DCI format scheduling a PDSCH providing a response to a RA from one or more UEs, the RNTI is a RA-RNTI. For a DCI format scheduling a PDSCH for Paging a group of UEs, the RNTI is a P-RNTI. For a DCI format providing TPC commands to a group of UEs, the RNTI is a TPC-RNTI. DCI formats with a SI-RNTI, or a RA-RNTI, or a P-RNTI, or a TPC-RNTI will be referred to as providing UE-common control information. Each RNTI type is configured to a UE through higher layer signaling (and the C-RNTI is unique for each UE).
FIG. 2 illustrates a conventional encoding process for a DCI format.
Referring to FIG. 2, a RNTI of a DCI format masks a CRC of a codeword in order to enable a UE to identify a DCI format type. The CRC 220 of the (non-coded) DCI format bits 210 is computed and it is subsequently masked 240 using the exclusive OR (XOR) operation between CRC and RNTI bits 220. This corresponds to XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, XOR(1,1)=0. The masked CRC bits are then appended to the DCI format bits 250, channel coding is performed 260, for example using a convolutional code, followed by rate matching 270 to allocated resources, and finally by interleaving and modulation 280, and transmission of a control signal 290. For example, both the CRC and the RNTI consist of 16 bits.
FIG. 3 illustrates a conventional decoding process for a DCI format.
Referring to FIG. 3, a received control signal 310 is demodulated and the resulting bits are de-interleaved 320, the rate matching applied at a NodeB transmitter is restored 330, and an output is subsequently decoded 340. After decoding, DCI format bits 360 are obtained after extracting CRC bits 350 which are then de-masked 370 by applying the XOR operation with a RNTI 380. Finally, a UE receiver performs a CRC test 390. If the CRC test passes, the DCI format is considered valid and a UE determines the respective parameters for a signal reception or a signal transmission. If the CRC test does not pass, a UE disregards the presumed DCI format.
A NodeB separately codes and transmits a DCI format in a respective PDCCH. To avoid a PDCCH transmission to a UE blocking a PDCCH transmission to another UE, the location of a PDCCH transmission in the time-frequency domain of a DL control region is not unique. As a consequence, a UE needs to perform multiple decoding operations to determine whether there is a PDCCH intended for the UE. The REs of a PDCCH are grouped into Control Channel Elements (CCEs) in the logical domain and include all REs of a DL control region except for REs used to transmit CRS, PCFICH, or PHICH. For a given number of DCI format bits, a number of CCEs for a respective PDCCH depends on the channel coding rate (Quadrature Phase Shift Keying (QPSK) is assumed as the modulation scheme). A NodeB may use a lower channel coding rate (more CCEs) for PDCCH transmissions to UEs experiencing low DL Signal-to-Interference and Noise Ratio (SINR) than to UEs experiencing a high DL SINR. The CCE aggregation levels can consist, for example, of 1, 2, 4, and 8 CCEs.
FIG. 4 illustrates a conventional transmission process of DCI formats in respective PDCCHs.
Referring to FIG. 4, encoded DCI format bits are mapped to PDCCH CCEs. The first 4 CCEs (L=4), CCE1 401, CCE2 402, CCE3 403, and CCE4 404 are used to transmit a PDCCH to UE1. The next 2 CCEs (L=2), CCE5 411 and CCE6 212, are used to transmit a PDCCH to UE2. The next 2 CCEs (L=2) CCE7 421 and CCE8 422, are used to transmit a PDCCH to UE3. Finally, the last CCE (L=1), CCE9 431, is used to transmit a PDCCH to UE4. The DCI format bits may be scrambled 440 by a NodeB-specific binary scrambling code and are subsequently modulated 450. Each CCE is further divided into Resource Element Groups (REGs). For example, a CCE consisting of 36 REs may be divided into 9 REGs each consisting of 4 REs. Interleaving 460 is applied among REGs (blocks of 4 QPSK symbols). For example, a block interleaver may be used to accomplish the interleaving 460. The resulting series of QPSK symbols may be shifted by J symbols 470 and each QPSK symbol is finally mapped to an RE 480. Therefore, in addition to the CRS, 491, 492, and 493, PCFICH and PHICH (not shown), a DL control region includes PDCCHs providing DCI formats to UE1 494, UE2 495, UE3 496, and UE4 497.
For the PDCCH decoding process, a UE may determine a search space for candidate PDCCH transmissions after it restores the CCEs in the logical domain according to a UE-common set of CCEs (Common Search Space or CSS) and according to a UE-dedicated set of CCEs (UE-Dedicated Search Space or UE-DSS). The CSS may consist of the first CCEs in the logical domain and is primarily used for transmitting PDCCHs for DCI formats providing UE-common control information, but it may also be used for transmitting PDCCHs for DCI formats providing UE-specific control information. The UE-DSS consists of all CCEs and is entirely used for transmitting PDCCHs for DCI formats providing UE-specific control information. The CCEs of a UE-DSS may be determined according to a pseudo-random function having, as inputs, UE-common parameters, such as the subframe number or the total number of CCEs in a subframe, and UE-specific parameters such as a C-RNTI. For example, for CCE aggregation level L ∈ {1,2,4,8} CCEs, the CCEs corresponding to PDCCH candidate m are given by Equation (1).CCEs for PDCCH candidate m=L·{(Yk+m) mod└NCCE,k/L┘}+i  (1)
In Equation (1), NCCE,k is the total number of CCEs in subframe k, i=0, . . . , L−1, m=0, . . . , Mc(L)−1, and Mc(L) is the number of PDCCH candidates to monitor in the UE-DSS. Exemplary values of Mc(L) for L ∈ {1,2,4,8} are, respectively, {6, 6, 2, 2}. For the UE-DSS, Yk=(A·Yk−1)mod D where Y−1=C-RNTI≠0, A=39827 and D=65537. For the CSS, Yk=0.
The DL control region in FIG. 1 is assumed to occupy a maximum of MsymbDL=3 subframe symbols and a PDCCH is transmitted substantially over an entire DL BW. This limits a PDCCH capacity of a DL control region and cannot support interference coordination in the frequency domain among PDCCH transmissions from different NodeBs or, in general, from different Transmission Points (TPs). Expanded PDCCH capacity or PDCCH interference coordination in the frequency domain is needed in several cases. One such case is a use of Remote Radio Heads (RRHs) in a network where a UE can receive DL signals either from a macro-NodeB or from an RRH. If the RRHs and the macro-NodeB share a same cell identity, cell splitting gains do not exist and expanded PDCCH capacity is needed to accommodate PDCCH transmissions from the macro-NodeB and the RRHs. Another case is for heterogeneous networks where DL signals from a pico-NodeB experience strong interference from DL signals from a macro-NodeB and interference coordination in the frequency domain among NodeBs is then needed.
Extending a conventional DL control region size to more than MsymbDL=3 subframe symbols is not possible at least because of a requirement to support conventional UEs which cannot be aware or support such extension. An alternative is to support DL control signaling in a conventional PDSCH region by using individual PRBs to transmit control channels. These control channels will be referred to as enhanced Control CHannels (eCCHs) and may include an enhanced PDCCH (ePDCCH), an enhanced PHICH (ePHICH), or an enhanced PCFICH (ePCFICH).
FIG. 5 illustrates a conventional allocation of resources for transmitting eCCHs in a DL TTI.
Referring to FIG. 5, although eCCH transmissions start immediately after the conventional CCHs 510 and are over all remaining subframe symbols, they may instead always start at a predetermined location, such as the fourth subframe symbol. The eCCHs are transmitted in four PRBs per subframe, 520, 530, 540, and 550 while the remaining PRBs per subframe are used for PDSCH transmissions 560, 562, 564, 566, 568.
A UE can be configured by higher layer signaling the PRBs that may convey eCCHs. The transmission of an eCCH to a UE may be in a single PRB, if a TP has accurate CSI for the UE and can perform Frequency Domain Scheduling (FDS) or beam-forming for the eCCH transmission, or it may be in multiple PRBs, if accurate CSI for a UE is not available or if an eCCH is intended for multiple UEs. An eCCH transmission over a single PRB will be referred to as localized or non-interleaved while an eCCH transmission over multiple RBs will be referred to as distributed or interleaved. Interleaved eCCHs may include ePDCCH, ePCFICH, or ePHICH while non-interleaved eCCHs may be only ePDCCHs.
The design of enhanced search spaces in a set of assigned PRBs is not material to the present invention and may follow, for example, conventional techniques. Then, an ePDCCH may consist of respective eCCEs and a number of ePDCCH candidates may exist for each eCCE aggregation level. An eCCE may or may not have the same size as a conventional CCE and an eCCE for non-interleaved ePDCCHs may or may not have the same size as an eCCE for interleaved ePDCCHs. Similar to CCEs, the eCCEs are distributed over a virtual BW formed by the set of assigned PRBs in the respective symbols of a DL TTI.
FIG. 6 illustrates conventional REs of a PRB for transmitting eCCHs.
Referring to FIG. 6, REs of a PRB 610 are allocated to transmission of eCCHs 620 and may also be allocated to transmissions of CCHs 630, CSI-RS 640, and CRS 650. A PRB assigned to transmissions of eCCHs also includes REs allocated to DMRS from different Antenna Ports (APs). In case of four DMRS APs, REs are assigned for DMRS transmission from a first AP 660, DMRS transmission from a second AP 670, DMRS transmission from a third AP 680, and DMRS transmission from a fourth AP 690.
A UE performs coherent demodulation of eCCHs using a channel estimate obtained by DMRS in same PRBs as the associated eCCHs. As DMRS transmitted in a same PRB in different subframes cannot be assumed to be the same, for example an associated precoding may not be the same or the PRB may not be assumed as being always used to transmit eCCHs, and as PRBs assigned to eCCHs are typically dispersed in frequency, a channel estimate for demodulating eCCHs may only be based on a DMRS in each PRB per subframe. Absence of interpolation in a time or frequency domain may significantly degrade the accuracy of DMRS-based channel estimates and the reception reliability of eCCHs compared to that of CCHs. This is because for a CCH demodulation, a UE may obtain a channel estimate from a CRS that is substantially transmitted over all subframes and over the entire operating BW thereby enabling time and frequency interpolation, respectively.
A same or different DMRS may be transmitted from each AP. A DMRS may also be scrambled by a sequence which can be TP-specific or UE-specific. An eCCH may also be scrambled by a sequence than can be TP-specific or UE-specific in order to enable TP-agnostic transmission of eCCHs and multiple TPs to transmit a same eCCH. However, the latter operation is only possible after a UE has established access to a network and it can then be configured by higher layer signaling, either explicitly or implicitly, a DMRS scrambling sequence or an eCCH scrambling sequence. Prior to establishing access to a network, a UE-specific DMRS or eCCHs scrambling sequence is not possible and a respective UE-common scrambling sequence is instead needed. Moreover, a UE needs to know PRBs and respective subframes where a TP transmits UE-common DCI.
Therefore, there is a need to enhance the accuracy of a channel estimate obtained from a DMRS for coherent demodulation of enhanced control channels.
There is another need to support DMRS scrambling with a UE-specific sequence and with a UE-common sequence associated, respectively, with enhanced control providing UE-dedicated control information and UE-common control information.
Finally, there is another need to define resources for transmitting enhanced control channels providing UE-common control information.